MATERIALS FOR A MONOGRAPH OP THE ASCONS. 583 



actiuoblast. ^^f^^r. a/). Gastral (internal) aperture of pore, gastr.ray. Gastral 

 raj of quadriradiate. 7i.Jl.ep. Nuclei of flat epithelium, por.c. Porocyte. 

 sex(. Sextet of spicule-formiug cells, spic. Spicule, spic. cell. Definitive 

 spicule-cell. spic.monax. Monaxon spicule, sp.sh. Spicule sheath, irirad. 

 si/st. Triradiate system, x. Rod-like body iu spicule-forming cells. 



PLATE 38. 



All the figures relate to Clathrina coriacea. 



Fig. 1. — Surface view from the gastral side of the dermal layer to show 



three cells of the flat epithelium that have migrated inwards (actinoblasts). 



The drawing further shows five cells of the flat epithelium, two spicule rays, 



oue of them bearing a spicule-cell, and a pore-cell. 



Pig, 2. — Surface view from the gastral side of dermal layer showing a 

 "trio " of actinoblasts near a spicule ray. Three nuclei of the underlying 

 flat epithelium are figured to show the relations of the cells. 



Pig. 3. — Surface view from dermal side of dermal layer showing a trio of 

 actinoblasts underneath a spicule ray. At a higher level are three cells of 

 the flat epithelium. 



Pig, 4. — A "sextet " showing three formative cells lying just above three 

 others. Prom the dermal aspect. 



Pig. 5. — A sextet in which the minute triradiate system has made its 

 appearance. 



Pig. 6.— a slightly older triradiate system in its sextet of formative cells. 

 Prom the gastral aspect. 



Pig. 7. — A further stage in the growth of the triradiate system seen from 

 the gastral side ; the three inner formative cells have travelled to the apices 

 of the rays, while the three outer ones remain at the base. 



Pig. 8. — Typical six-celled stage in the formation of the triradiate system, 

 seen from the dermal aspect; the three irregularly shaped inner formative 

 cells at the apices of the rays, the three spindle-shaped outer formative cells 

 at the bases of the rays. 



Pig. 9. — Triradiate system in which two apical formative cells have dis- 

 appeared, one still remaining. Dermal aspect. 



Pig. 10.— View of dermal layer after removal of collar-cells, showing 

 various cell elements from the dermal aspect. The drawing shows ten cells of 

 the flat epithelium (/. ep.), six pore-cells {por. c), one actinoblast {act. hi.), 

 one young triradiate system bearing three basal formative cells and a sinMe 

 apical formative cell {apic.form. cell) persistent on one ray ; overlying this the 

 ray of a fully formed triradiate system, bearing a spicule-cell {spic. cell), and 

 finally one granular (nutritive) wandering cell {am.c}) and five non-granular 

 wandering cells {am.c.^). 



